A study will be made of the role of brain protein synthesis in the development of sensitivity to the stimulant actions of morphine in mice. We will attempt to correlate inhibition of brain protein synthesis in vivo with blockade of sensitization by cycloheximide. The possible relationship of long-lasting analgesic tolerance and protracted abstinence will be investigated. Brain levels of serotonin, nor- epinephrine and dopamine by treatment with MAO inhibitors and inhibitors of neurohormone synthesis, such as p-chlorophenylalanine, in an attempt to relate stimulation by morphine to changes in hormone metabolism. We will also examine the effect of morphine pretreatment on the amount of narcotic-receptor protein in the whole brain and in selected regions such as the caudate nucleus and hypothalamus.